1. Field of the Invention
The present invention relates to an improvement in solid state color imaging apparatus, especially in the circuit configuration of the solid state color imaging apparatus.
2. Description of the Prior Arts
As is well known, there are two types of solid state color imaging device these are charge-coupled devices (hereinafter is referred as CCD) which is a device for transferring electric charges, and bucket-brigade devices (hereinafter is referred as BBD) MOS types device of X-Y address devices also exist. The above-mentined MOS type solid state imaging apparatus is constituted in a manner output an image signal by sequentially switching MOS type transistors by means of a vertical shift register and a horizontal shift register. Accordingly, spike noises due to switching pulses by horizontal scanning is liable to be generated. These spike noises increase as the number of picture elements increase, thereby limiting a S-N ratio of the image.
A solid state imaging device of X-Y address type reads out signal charge from the picture element by addressing. Accordingly there is no crosstalk of signal charge in vertically neighboring picture elements. Accordingly, such X-Y address type solid state imaging device is suitable in a system for obtaining a color signal by utilizing vertical correlation of picture element signals using the mosaic shaped color filter as shown in FIG. 1, wherein dotted line squares show photodiodes of light receiving parts and solid line squares show color filter elements.
Among the CCDs, there are two types. These are frame transfer type CCD in which signal charge of the light receiving parts are transferred to a storing part with a high speed, and the stored signal is sequentially read out through horizontal transferring means. Also, an interline type can be used wherein the signal charge of the photodiodes on corresponding vertical columns the read out respectively to are vertical transfer lines. The read-out signal charges are further transferred to horizontal transfer means within a subsequent horizontal scanning time, and the signal charges are taken out from the horizontal transfer means in sequence. The former frame transfer type CCD apparatus is not usable in a solid state color imaging apparatus, wherein the color signals are taken out by utilizing vertical correlation, since that type apparatus has a channel stopper only in a vertical direction, and crosstalk of signal charge is generated in vertically neighboring picture elements. Accordingly, it has been necessary in the art to provide an improved system having a stripe-shaped color filter with R-stripes, G-stripes and B-stripes as shown in FIG. 2 thereby to eliminate color distortion even when there is crosstalk. This prior art apparatus has a problem that the band-width of the luminance signal can not be sufficiently wide since spatial repetition period of the color filter is determined by the length of three picture elements.
In the latter interline type apparatus, even a mosaic shaped color filter as shown in FIG. 1 can be used since the photodiodes are disposed isolated corresponding to the spatial area of the color filter elements. However, it has a problem that transfer efficiency of the signal charge in the vertical transfer means can not achieve 100%, and impurities of colors due to residual signal charges in the vertical transfer means are produced.
In order to solve the above-mentioned problems, an improved solid state color imaging apparatus is herein disclosed, wherein effect to picture quality of non-transfer efficiency in vertical transfer means of solid state imaging device is eliminated, lowering of horizontal resolution due to color filter repetition cycle in horizontal direction is prevented and efficiency of light utility is high.
FIG. 3 and FIG. 4 show electrical and filter-disposition configuration for such apparatus.
FIG. 3 shows one example of a circuit connection of the general color solid state imaging sensor of a charge priming transfer type. In FIG. 3, numeral 1 designates a solid state imaging sensor made as a monolithic IC, wherein 2a, 2b, and 2c designate photodiodes. Generally, these photodiodes 2a, 2b, 2c . . . are made using P-N junctions. The centers of these diodes 2a, 2b, 2c . . . are disposed in a manner that photodiodes of one horizontal line are disposed in interleaving positions with respect to vertically neighboring horizontal lines, thereby forming a check pattern like disposition. Vertically disposed signal lines 3a, 3b, 3c . . . are connected to the photodiodes in a manner that photodiodes of one horizontal line are connected to one every other vertically disposed signal lines 3a, 3c, 3e . . . or 3b, 3d . . . . That is, in FIG. 3, the first photodiode 2a of the first horizontal line is connected to the first signal line 3a, the first photodiode 2b of the second horizontal line is connected to the second signal line 3b, and so on, respectively through vertical switching MOS FETs 4a, 4b, 4c . . . .
Vertical shift register 5 has input terminals for vertical scanning pulse .phi..sub.SP and input terminals for clock pulses .phi..sub.V1, .phi..sub.V2, and output terminals of respective stages of the vertical shift register 5 are connected through vertical address lines 6a, 6b, 6c to the gates of MOS FETs 4a, 4a' . . . , 4b, 4b' . . . , 4c, 4c' . . . , 4d, 4d' . . . . Each end of the signal lines 3a, 3b, 3c . . . is connected to the sources 7a, 7b, 7c . . . and 7a', 7b', 7c' . . . of transfer MOS FETs. Gates of these transfer MOS FETs are connected in common to transfer pulse input line .phi..sub.TG. Drains of the MOS FETs are commonly connected to each end of transfer capacitors 8a, 8b, 8c . . . and 8a', 8b', 8c' . . . , the other ends of which are connected commonly to transfer pulse input line .phi..sub.TC. Here, the capacitances of the transfer capacitors 8a, 8b . . . , 8a', 8b' . . . are sufficiently smaller than those of the vertical disposed signal lines 3a, 3b, 3c . . . .
The drains of the of transfer MOS FETs 7a, 7b . . . 7a', 7b' . . . are connected to another transfer gate devices 9a, 9b, 9c . . . and 9a', 9b', 9c' . . . , respectively. Control electrodes of these FETs are connected commonly to transfer gate input line .phi..sub.TB. Output terminals of the gating devices 9a, 9b . . . and 9a', 9b' . . . are connected to charge-coupled type horizontal shift registers (hereinafter are referred as horizontal CCD) 10a and 10a', respectively. Output ends of the horizontal CCDs are connected to signal output parts 11a and 11a', respectively. The operation of the Circuit of FIG. 3 will now be discussed.
During one vertical scanning period, incident light from an object signal charges to accumulate in the photodiodes 2a, 2a' . . . , 2b, 2b' . . . , 2c, 2c' . . . . By impressing vertical scanning pulses produced by the vertical shift register 5 to the gates of vertical switching MOS FETs 4a, 4a' . . . , 4b, 4b' . . . 4c, 4c' . . . , signal charges stored in the photodiodes 2a, 2a' . . . , 2b, 2b' . . . 2c, 2c' . . . are transferred onto the vertically disposed signal lines 3a, 3b . . . . Then, by impressing signal voltages on the input lines .phi..sub.TG and .phi..sub.TC, the signal charges which have been transferred to the vertically disposed signal lines 3a . . . are further transferred to the transferring capacitance devices 8a, 8b . . . , 8a', 8b' . . . . By impressing signal voltage on the transfer gates 9a, 9b . . . , 9a', 9b' . . . , the signal charges transferred in the capacitance devices 8a, 8b . . . , 8a', 8b' . . . are all transferred to the horizontal CCDs 10a and 10a'. Then, the signal charges transferred to the horizontal CCDs 10a and 10a' are read out to the signal output parts 11a and 11a' by applying an appropriate transferring clock signals to the signals output parts 11a during one horizontal scanning period. The clock frequency of the horizontal transferring is determined by the numbers of photodiodes 2a, 2a' . . . , 2b, 2b' . . . 2c, 2c' . . . , respectively in each horizontal line. When, for instance, 384 photodiodes are disposed in one horizontal line, the clock frequency is about 7.2 MHz.
The solid state imaging sensor, which operates in the above-mentioned principle, is also characterized by reading out signals of plural horizontal lines at a same time within one horizontal scanning period. That is, in the n-th horizontal scanning of the first field, the device simultaneously reads out signals of the two horizontal lines 2a, 2a' . . . and 2b, 2b' . . . . In an (n+1)-th horizontal scanning of the first field, the device simultaneously reads out signals of the two horizontal lines 2c, 2c' . . . and 2d, 2d' . . . , and further in the second field, reads out signals of the two horizontal lines 2d, 2d' . . . and 2e, 2e' . . . .
FIG. 4 schematically shows one example of a configuration of the color filter for the solid state imaging sensor. In FIG. 4, squares shown by dotted lines schematically designate photodiodes 2a, 2a' . . . , 2b, 2b' . . . , 2c, 2c' . . . , and stripe filters consisting of regular repetition of cyan (Cy) filter 12, yellow (Ye) filter 13 and magenta (M) filter 14. In the disposition, a spatial repetition period of the stripe filter and the photodiodes are equal.
By making the above-mentioned arrangement of the photodiodes 2a . . . 2b . . . 2c . . . and the stripe color filters, in the n-th horizontal scanning, output signals of the photodiodes 2a, 2a', 2a" . . . , namely a signal which is spatially modulated by the Cy, Ye and M filters, is produced at the output terminal of the horizontal CCD 11a, and on the other hand, at the output terminal of the horizontal CCD 11a' a signal of the photodiodes 2b, 2b', 2b", namely stripe filters Ye, Cy and M is produced.
In case an operation of the solid state imaging apparatus is made by applying the stripe color filters of the configuration shown in FIG. 4 on the solid state imaging sensor of FIG. 3, signal charges of the horizontal lines 2a, 2a' . . . and 2b, 2b' . . . shown in FIG. 3 are transferred by horizontal CCDs 10a and 10a' which are operated with a predetermined phase difference at the same time. That is, the output of the CCDs 10a and 10a' are obtainable at the output terminal of their output parts 11a, 11a' in dot sequential manner. Generally, phase differences between the transferring pulses of the first horizontal CCD 10a and the second horizontal CCD 10a' are selected to be separated by 180.degree. of phase from each other. Relative phases and spectral characteristics of the output signal of the horizontal CCDs 10a and 10a' are operated in the above-mentioned way are shown in FIG. 5(A) and FIG. 5(B).
FIG. 5(A) shows the output signal of the first horizontal CCD 10a, namely the output signal of the photodiodes 2a, 2a', 2a" . . . , and FIG. 5(B) shows the output signal of the second horizontal CCD 10a, namely the output signal of the photodiodes 2b, 2b', 2b" . . . .
In FIG. 5, a time period between neighboring color signals is 1/f, where f is horizontal transfer clock frequency. And previously stated, the signals of FIG. 5(A) and FIG. 5(B) have 180.degree. phase difference from each other. In FIG. 5(A) and FIG. 5(B), names of colors mentioned in the squares designate primary colors and names of colors mentioned on the square blocks colors of the stripe filters, colors of which contain the primary colors as a component thereof. This is based on the relationship that, when the intensities of red color, green color and blue color are R, G, B, respectively, then: EQU Cy=(G+B),M=(R+B),Ye=(R+G) (1)
FIG. 6(A) and FIG. 6(B) show spectral distributions of the signals shown by FIG. 5(A) and FIG. 5(B), wherein frequency f is a horizontal transfer clock frequency and a modulated chrominance signal is generated to have spectral distribution around a color carrier which is 1/3f. The phase of the color carriers in this case are opposites as shown by FIG. 7(A) and FIG. 7(B). As shown by FIG. 7(A) and FIG. 7(B), by adding the output signals of CCD 10a and CCD 10a', modulated chrominance signal cancel each other. Furthermore, by the adding, the signals shown in FIG. 8, namely an effective number of photodiodes in one horizontal line is equivalently doubled, and the sampling frequency in horizontal direction is equivalently substantially doubled. FIG. 9 shows a spectral distribution of signals shown in FIG. 8. As is obvious from FIG. 8 and FIG. 9, by adding output signals of the horizontal CCD 10a and CCD 10a', the modulated chrominance signal, which is around 1/3f frequency, is off-set, and accordingly, the modulated chrominance signal is produced only around a color carrier signal of 2/3f. That is, the actual band width of the luminance signal is limited to a width of 2/3f. However, the value of 2/3f is twice the conventional band width 1/3f of the luminance signal, and therefore the horizontal resolution is greatly improved.
The above-mentioned explanation is made for the n-th horizontal line, that is the lines a and b. A similar explanation applies for the (n+1)-th lines, namely horizontal lines c and d. Furthermorre, for n-th line of the second field, namely horizontal lines b and c, the same apply.
Nextly, an embodiment of electric signal processing circuit of the color solid state imaging is described with reference to FIG. 10.
In FIG. 10, numeral 15 designates a solid state imaging sensor with stripe color filters as described above. A synchronization signal generator 16 produces a synchronization signal to a driving circuit 17 which produces the above-mentioned signals .phi..sub.SP, .phi..sub.V1, .phi..sub.V2, .phi..sub.TG, .phi..sub.TC and .phi..sub.TB for driving the solid state imaging sensor 15. Two output signals of the solid state imaging sensor 15 are fed to an adder 18 for being added to each other, and the added output signal is fed through a low pass filter 19 having a pole frequency of 2/3f, producing a luminance signal to an encoder 20. On the other hand, the output of the adder 18 is also given through a band-pass filter 21 having a center frequency of 2/3f, putting out a modulated chrominance signal to synchronized, of synchronous detectors 22 and 23 which detect, using synchronization detection, reference signals having 90.degree. phase differences. The output signals of the synchronization detectors 22 and 23 are passed through low-pass filters 24 and 25 for removing unnecessary high frequency range parts to the encoder 20 to compose a color television signal. The frequencies of the synchronization detection reference signals to be fed to the synchronous detectors 22 and 23 have a frequency of 2/3f and are synchronized with the horizontal transfer clock signal. A phase shifter 26 is provided to give a 90.degree. phase shift between the synchronization detection reference signals to the synchronous detectors 22 and 23.
FIGS. 7(A), (B) are diagrams showing the phase of the output signal from the CCDs 10a and CCD 10a'. The two signals are expressed as S.sub.ca and S.sub.ca '. Thus, expressing the vector diagrams mathematically: ##EQU1##
Here, .omega. is 2.pi.1/3f.
S.sub.ca and S.sub.ca ' show the modulated chrominance signal components.
The signal wave in FIG. 8 is obtained by adding the two kinds of signals shown in FIG. 5. The frequency of the modulated chrominance signal in FIG. 8 is twice the frequency of the modulated chrominance signal in FIG. 5.
Then, when the levels of the Cy, Ye and M are equal, the modulated chrominance signal component is zero.
The actual modulated chrominance signal Sc of the output signal of the solid state imaging sensor is given as follows as a general formula.
Provided that output signal of the band-pass filter is Sc, the folowing equation holds as is obvious from FIG. 8 and FIG. 9: ##EQU2##
Then, by synchronizedly detecting the above-mentioned signal Sc by utilizing the reference signals cos 2.pi..2/3 ft and sin 2.pi..2/3 ft, then color difference signals of ##EQU3## are obtainable as the output signals of the low-pass filter 24 and low pass filter 25. By modulating the color sub-carrier by utilizing these two color difference signals, and by combining the luminance signal thereto, an NTSC composite color television signal is obtainable.
In this color separation system, the color carrier signal appearing at 1/3f frequency is eliminated by off-setting by utilizing vertical correlation (adding of simultaneously read out signals for two lines).
Accordingly, the above-mentioned conventional system has such problem that when a part of the object having no vertical correlation thereat is photographed, there is a possibility of producing some noises in the luminance signal and chrominance signal as follows:
Firstly, the noise in the luminance signal to be produced when the part having no vertical correlation is explained.
FIG. 11 schematically shows one example of an objective image projected on the solid state imaging sensor and locations of photodiodes of the solid state imaging sensor when there is no vertical correlation between the horizontal line signals. In this example, the objective image changes between the upper horizontal line a and lower horizontal line b which together constitute an n-th horizontal line for the signal. Also, the objective image changes between the upper horizontal line e and lower horizontal line f which together constitute an (n+2)-th horizontal line for the signal. That is, the area on and above the line a is red and the area of line b and below until the line e is white, and the bottom area of line f and below is black. Schematic signal charts of the output signal of the adder 18 and the horizontal scanning output signals for n-th line and (n+2)-th line are shown in FIG. 12(A) and FIG. 12(B). As is shown in FIG. 12(A), the signal has one blank time every six picture element scanning times, and therefore, this blank results in producing modulated chrominance signal equivalently having frequency component of 1/3f. Since this modulated chrominance signal is within the band-width of the luminance signal, this becomes an interference signal, i.e., noise to deteriorate picture quality. That is, in a TV image, a noise induced by the 1/3f modulated chrominance signal is produced on the scanning line of the n-th part where the vertical correlation does not exist, thereby producing horizontal dot line noise between the different color regions as shown in FIG. 13. In an actual case, the dotted line is induced by 2.4 MHz signal.
Nextly, an example where no vertical correlation in the horizontal signals is produced in a colorless object is discussed. As shown in FIG. 12(B), when the objective image turns from white to black between the upper line e and the lower line f of an (n+2)-th horizontal line, the white incident light causes generation of signals for R, G and B, but no modulated chrominance signal is generated, since the level of the output signals for the R, G and B color are uniform. However, when color temperature of the objective image changes once, the relative energy to the incident light wavelength changes, and therefore, output singal levels for the R, G and B color filter parts are not uniform as shown in FIG. 12(C). Therefore, another color carrier having the phase and amplitude shown by arrow R' as shown in FIG. 12(D) of the frequency of 1/3f is generated within the frequency range of the luminance signal, and produces a interference signal for the reproduced image. It is very difficult to retain the color temperature constant, even if a color conversion filter is used for all the range of the brightness of area. Therefore, unless the brightness is limited within a very strictly narrow range in order to keep the color temperature balance, there is also a liability of producing dotted line noise as shown in FIG. 13.
Nextly, description is made of chrominance signals for the parts where no vertical correlation of the horizontal signals exist.
FIG. 14 schematically shows a case of an objective image and photodiodes with color filters. In this case, between the horizontal line a and horizontal line b which together constitute an n-th horizontal scanning line, the objective image vertically changes, and also between the horizontal line e and horizontal line f, which together constitute an (n+2)-th horizontal scanning line, the objective image vertically changes. That is, on and above the horizontal line a, the objective image is white, and from line b to line e the objective image is red, and from line f and downward the objective image is green. FIG. 15(A) and FIG. 15(B) shown the output signals of the adder 18 for the scannings of n-th line and (n+2)-th line, respectively. As is obvious from FIG. 15(A), in the signal components obtained from the repetition of photodiodes with six color filters, the intensities of the signal components for red (R), green (G) and blue (B) are R:G:B=4:2:2, and accordingly the chrominance signal of the n-th horizontal line yields a reddish signal, even though there is no actual reddish part in the image. As is obvious from FIG. 15(B), the signal components obtained from the repetition of photodiodes with six color filters, the intensities of the signal components for red (R), green (G) and blue (B) are R:G:B=2:2:0, and accordingly chrominance signal of the (n+2)-th horizontal line is yellowish signal. In this latter case, though the object turns from red to green and no yellowish color existed in the object, the reproduced television image has a yellow horizontal line at the boundaries between the red region and the green region, and the color quality is deteriorated.
As has been described, when signal correlation in vertical direction is zero or small, undesirable dot noise is produced in the luminance signal at that part, and also an undesirable spurious color signal is produced at the part, both deteriorating the image quality.